Disruption of the normal patterns of phosphorylation results in aberrant regulation of signal transduction and has been implicated in the etiology of a variety of major human diseases, including cancer, diabetes and obesity. The ability to modulate signaling pathways selectively, such as through inhibition of protein tyrosine kinases (PTKs), holds enormous therapeutic potential; nevertheless, there remain challenges, in particular drug resistance to the drug. Therefore, despite the obvious potential of targeting PTKs, it is anticipated that alternative strategies, to target simultaneously different signaling enzymes and processes, would be more effective than targeting individual PTKs alone. Consequently, a major problem remains the identification of targets for such alternative therapies. The broad, long-term objectives of this grant, which is currently in Year 23, have been to characterize the structure, modes of regulation and physiological function of members of the protein tyrosine phosphatase (PTP) family of enzymes. In particular, the focus has been on establishing links between disruption of PTP function and the etiology of human disease, with a view to identifying and validating new therapeutic targets and strategies from among the PTPs themselves or the constituents of the signaling pathways that they regulate. In the previous funding period, we established cell and animal models to investigate new functional links between particular PTPs and specific signaling pathways in breast cancer. RNAi-based screens were completed successfully and have now focused attention on particular PTPs for mechanistic studies in this competing renewal. A highlight of the previous funding period was the development of a small molecule drug candidate that inhibits PTP1B by a novel allosteric mechanism, targeting preferentially the full-length form of the phosphatase that contains the regulatory C-terminal segment of the protein. This basic research has moved the field forward substantially by establishing PTP1B as a bona fide target for therapeutic intervention in HER2-positive cancer and forming the basis for a clinical trial in HER2-positive breast cancer patients. Going forward, the goal is to exploit these strategies further both to use PTP1B inhibitors as a novel way to address the major problem of resistance to signal transduction-based therapies and also to harness PTP1B inhibition as a new approach to developing therapies for other HER2- positive cancers, such as gastric cancer, for which treatments are desperately lacking. Furthermore, the studies will validate additional novel targets from among the members of the PTP family for therapeutic intervention in cancer, with a particular emphasis on PTPs that function positively to promote signaling. To achieve this goal, I have assembled a strong team of collaborators, with a long history of working together in this area, and will address the following Specific Aims: Specific Aim 1: To define the signaling function of PTP1B in HER2-positive cancer and exploit further its potential as a therapeutic target. Specific Aim 2: To define the function of receptor PTPd as a potential therapeutic target in tumors characterized by loss of Missing-in-Metastasis. Specific Aim 3: To define the phosphatidic acid-protein tyrosine phosphatase PTPD2 signaling axis in the HER2 pathway in mammary epithelial cells. Specific Aim 4: To define how loss of expression of specific PTPs that regulate cell migration and invasion in vitro contribute to tumorigenesis in vivo.